Rationale:

When adding spots or driving lamps which will be used along with the existing headlamps, the original wiring will need to be modified or extended slightly. Usually additional lights are added so that they are powered via a relay, but there are very good reasons to convert the existing headlamps to use relays at the same time. Powering bright lamps at 12v requires quite a bit of power, all of which travels through the wiring behind the dash. High powers (hence high amperages) equal possible component failures and a very real fire risk. Old Lucas (or equivalent) dashboard or column switches can easily burn out, even at 'book' power levels, and overheated cable can easily start smoking and flame. There are more reasons too -including the size and tidiness of wiring - but you get the idea.

So... If relays are used, then all the high power wiring from the battery to the lamps themselves remains under the bonnet, and only the relay coil supply wiring it taken into the car. With the wiring diagram above, the 3 relays plus a small lamp in the 'spots switch' consume about 500ma max. Without relays we'd be talking 20A or more when both main-beams and driving lamps were switched on.

Components used:

Wiring: The wiring diagram uses wiring colours as used in a GT6 Mk1. Dipped beams = blue/red, Mains = blue/white, so it seemed logical to use blue/yellow wiring for the additional lamps as it doesn't appear elsewhere on the car.
When buying wiring I've used modern thin-wall stuff which has a thin very flexible but strong wall and allows higher rating cable to occupy less space.

Relays: As the relays are performing simple on/off switching only (no change over or latch operations) I've used 3 SPNO 12V30A types. Most have a coil resistance of about 80 ohm so at 12v thats 150ma each. If SPNO types are not easily available then SPCO or DPNO type will be ok, you'll just leave a few terminals disconnected. Obviously any relay you use will need to be able to handle the sort of current your passing through it.
e.g. A relay capable of switching 30A is ok for use with two 150w spots, or four 80w spots - but if you're going that high DON'T FORGET to re-spec the wire ratings!

Fuses: I'm including fuses for each pair of lamps. Some people fuse each lamp but this might be overkill; I figure that if I lose any one pair, I still have the other two to get me home. Its VERY important to include some sort of fusing in the high current parts of the circuit becasue if there is a short circuit somewhere, you don't want melting cables, flat batteries and fire risks.
Fuse Ratings... The fuse needs to be rated higher than the load its feeding, but lower than the rating of any cabling its wired inline with. It has to blow first if there is a short of any kind.

To be continued...